I. Technical Field
The present invention relates to a seal structure used in a rotating shaft portion of a rotary machine.
II. Description of the Related Art
In a rotary machine, such as a steam turbine, a gas turbine, a compressor, etc., a so-called labyrinth seal structure is widely used as a seal structure for a rotating shaft portion.
A labyrinth seal structure is constituted of fins that are provided in multiple stages in the axial direction on a rotating shaft or a stationary portion facing it, protruding in a ring-like manner, and a surface that opposes these fins (opposing surface).
Because sealing performance, in other words, the performance of a rotary machine, is determined by the number of fins and a gap between the fins and the opposing surface, in order to enhance the performance of a rotary machine, there is a need for reducing the gap between the fins and the opposing surface.
Because a rotary machine is generally designed such that a rotating shaft rotates stably in a rated rotation speed range, while the rotation speed is increasing immediately after start-up, there is a speed range within which the vibration level of the rotating shaft reaches a maximum (hereinafter, this is referred to as “critical speed range”). The rotating shaft reaches the rated rotation speed range (rated operation) via this critical speed range.
In addition, where high-temperature steam is handled, for example, in a steam turbine, etc., the rotating shaft and the stationary portion thermally expand due to the high-temperature steam introduced after start-up; however, because, initially, there is large temperature unevenness in individual portions, partial differences occur in the thermal expansion of the rotating shaft and the stationary portion. Accordingly, because the rotating shaft and the stationary portion relatively move in the axial direction and the radial direction, the gap between the fins and the opposing surface narrows, posing the risk of contact between the fins and the opposing surface. When the fins come into contact with the opposing surface, the fins and the opposing surface are abraded, causing the set gap between the fins and the opposing surface during the rated operation to become larger than an initial setting; therefore, sealing performance deteriorates by a corresponding amount.
Because the gap between the fins and the opposing surface is set such that the fins do not come into contact with the opposing surface even with a vibration level in this critical speed range, it has not been possible to set a very small gap during rated operation.
As a countermeasure, for example, as disclosed in Japanese Unexamined Patent Application, Publication No. 2000-97352, a configuration that makes set gaps between fins and an opposing surface differ during start-up/shut-down and during the rated operation is used; that is, the gap is made large during start-up/shut-down to ensure safety and is made small during the rated operation to enhance the sealing performance.
This is achieved by a movable seal ring, in which a portion of a seal ring provided with fins mounted on a stationary portion is made radially movable and constantly biased outward by an elastic piece. In addition, an inner circumferential surface of the movable seal ring receives outward pressure from the passing working fluid. This pressure gradually decreases toward a low-pressure portion side. The working fluid of a high-pressure portion flows to an outer circumferential surface of the movable seal ring, and the movable seal ring is pressed inward with that pressure.
During a period in which the load is small and a pressure difference between the high-pressure portion and the low-pressure portion is small, as in during start-up/shut-down, because a pressure difference of the working fluid acting on the outer circumferential surface and the inner circumferential surface of the movable seal ring is not large, the movable seal ring is positioned at the outer side by a biasing force of the elastic piece. In other words, a state in which the set gap between the fins and the opposing surface is large is maintained.
When the load of a steam turbine increases, the pressure difference of the working fluid acting on the outer circumferential surface and the inner circumferential surface of the movable seal ring correspondingly increases; therefore, the movable seal ring moves inward by overcoming the biasing force of the elastic piece. Then, as the load increases, the movable seal ring moves to the inner end of a moving range. In other words, a state in which the set gap between the fins and the opposing surface is small is maintained.
In addition, a seal structure in which a coating of an abradable material that generates a small amount of heat during contact is formed on an opposing surface is also employed. Because the influence thereof at the time of unforeseen contact can be alleviated by the coating, this allows further reduction of the gap between fins and the opposing surface for enhancing the sealing performance.